The present invention relates to an implantable medical device and, more particularly, an implantable stent that includes substantially radiopaque ends and a method of using the same in the prevention of restenosis.
Stenosis is a narrowing or constriction of a duct or canal. A variety of disease processes, such as atherosclerotic lesions, immunological reactions, congenital abnormalities, and the like, can lead to stenosis of arteries or ducts. In the case of stenosis of a coronary artery, this typically leads to myocardial ischema. Percutaneous transluminal coronary angioplasty (PTCA), the insertion and inflation of a balloon catheter into a stenotic vessel to affect its repair is widely accepted as an option in the treatment of obstructive coronary artery disease. In general, PTCA is used to increase the lumen diameter of a coronary artery partially or totally obstructed by a build-up of cholesterol fats or atherosclerotic plaque. Typically a first guidewire of about 0.038 inches in diameter is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the first guidewire to a point just proximal of the stenosis. The first guidewire is then removed. A balloon catheter on a smaller 0.014 inch diameter second guidewire is advanced within the guiding catheter to a point just proximal of the stenosis. The second guidewire is advanced into the stenosis, followed by the balloon on the distal end of the catheter. The balloon is inflated causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten reclosure of the dilated vessel or even perforations in the vessel wall.
Other vascular invasive therapies include atherectomy (mechanical systems to remove plaque residing inside an artery), laser ablative therapy and the like. While the stenosis or occlusion is greatly reduced using these therapies, including PTCA, many patients experience a reoccurrence of the stenosis over a relatively short period. Restenosis, defined angiographically, is the recurrence of a 50% or greater narrowing of a luminal diameter at the site of a prior coronary artery disease therapy, such as a balloon dilatation in the case of PTCA therapy. Additionally, researchers have found that angioplasty or placement of a stent in the area of the stenosis irritates the blood vessel causing rapid reproduction of the inner layer of blood vessel cells and restenosis through a mechanism called hyperplasia. Restenosis is a major problem which limits the long-term efficacy of invasive coronary disease therapies. In particular, an intra-luminal component of restenosis develops near the end of the healing process initiated by vascular injury, which then contributes to the narrowing of the luminal diameter. This phenomenon is sometimes referred to as xe2x80x9cintimal hyperplasia.xe2x80x9d In some instances, restenosis develops so rapidly that it may be considered a form of accelerated atherosclerosis induced by injury. Additionally, the rapid onset of restenosis is compounded by the lack of predictability to determine which patients, vessels, or lesions will undergo restenosis.
Although the mechanism of restenosis is not fully understood, clinical evidence suggests that restenosis results from a migration and rapid proliferation of a subset of predominately medially derived smooth muscle cells which is apparently induced by the injury caused by the invasive therapy. Such injury, for example, is caused by the angioplasty procedure when the balloon catheter is inflated and exerts pressure against the artery wall, resulting in media tearing. It is known that smooth muscle cells proliferate in response to mechanical stretch and stimulation by a variety of growth factors. It is believed that such proliferation stops one to two months after the initial invasive therapy procedure but that these cells continue to express an extracellular matrix of collagen, elastin and proteoglycans. Additionally, animal studies have shown that after balloon injury, denudation of endothelial cells occurs, followed by platelet adhesion and aggregation, and the release of platelet-derived growth factor (PDGF) as well as other growth factors. As mentioned above, this mass of tissue contributes in the re-narrowing of the vascular lumen in patients who have restenosis. It is believed that a variety of biologic factors are involved in restenosis, such as the extent of the injury, platelets, inflammatory cells, growth factors, cytokines, endothelial cells, smooth muscle cells, and extracellular matrix production, to name a few.
Implantable devices are known for use in the prevention of restenosis. Typically, such implantable devices are intra-arterial stents fabricated from either a pure metal or a metal alloy. In general, a biocompatible metal stent props open blocked coronary arteries, keeping them from reclosing after balloon angioplasty. For example, a stent is described by Wiktor in U.S. Pat. No. 4,886,062, wherein the stent includes metal elements made of wire loops in a wound structure which allows individual loops to move with respect to one another. Another stent is described by Wolff in published European Patent Application No. 0421729, wherein the stent includes metal elements made of individual stent segments joined together by hinges.
These stents can then be deployed in a body lumen by means appropriate to their design. First, a balloon of appropriate size and pressure is first used to open the lesion. Then, for example, in the case of the stent described by Wiktor, the process is repeated with a stent crimped on a second balloon. The second balloon may be a high pressure type of balloon, e.g., more than 12 atmospheres, to insure that the stent is fully deployed upon inflation. The stent is deployed when the balloon is inflated. The stent remains as a permanent scaffold after the balloon is withdrawn. A high pressure balloon is preferable for stent deployment because the stent must be forced against the artery""s interior wall so that it will fully expand thereby precluding the ends of the stent from hanging down into the channel encouraging the formation of thrombus.
Thus, there is a continuing need for improved implantable devices that can be used in preventing restenosis. In particular, it would be desirable to provide an implantable device that can be easily and accurately inserted into even very small vessels and which accurately center the source in the vessel while permitting effective perfusion so that treatment can be conducted over reasonably long periods.
It is an object of the invention to provide an implantable device for preventing restenosis. It is also an object of the present invention to provide an implantable device that can be easily and accurately deployed in a body lumen by providing a visible means to assess deployment and placement of the implantable device. It is a further object of the present invention to provide an implantable device having longitudinal flexibility which allows it to conform to curves and variations within a body lumen and thus is less traumatic to the in vivo implantation site.
These and other objects have been accomplished by the implantable device of the present invention. One aspect of the present invention provides an implantable medical device including a radiolucent midsection tubular structure having a first end and a second end, the midsection tubular structure being expandable from an initial diameter to a second diameter; and a first tubular structure attached to the first end of the midsection tubular structure, the first tubular structure being expandable from an initial diameter to a second diameter to provide a substantially uninterrupted passageway therethrough, wherein the first tubular structure comprises a substantially radiopaque material.
Another aspect of the present invention provides an implantable medical device including a midsection tubular structure having a first end and a second end, the midsection tubular structure being expandable from an initial diameter to a second diameter, wherein the midsection is formed from a substantially radiolucent material. The implantable device also includes a first tubular structure attached to the first end of the midsection tubular structure, the first tubular structure being expandable from an initial diameter to a second diameter; and a second tubular structure attached to the second end of the midsection tubular structure, the second tubular structure being expandable from an initial diameter to a second diameter to provide a substantially uninterrupted passageway from the first tubular structure to the second tubular structure, wherein the first and the second tubular structure each consist essentially of a substantially radiopaque material.
Preferably, the substantially radiopaque material is more compliant than the substantially radiolucent material. Typically, the radiopaque material has a density of about 19.3 gm/cm3 to about 21.0 gm/cm3.
The radiopaque material is preferably selected from the group consisting of tantalum, gold, iridium, and a combination thereof, whereas the radiolucent material is preferably selected from the group of stainless steel, niobium, titanium, and a combination thereof.
Preferably, in one embodiment, the first tubular structure comprises less than about 25% of a total length of the implantable device. Preferably, in another embodiment, the first tubular structure and the second tubular structure each comprise about 20% to about 15% of a total length of the implantable device.
Yet another aspect according to the present invention provides a method for the treatment of restenosis which includes placing an implantable device having an initial diameter on a catheter suitable for delivery in a body lumen to form a catheter/device assembly. Preferably, the implantable device includes an expandable midsection tubular structure having a first end and a second end; and an expandable first tubular structure attached to the first end of the midsection tubular structure, wherein the first tubular structure comprises a substantially radiopaque material. The method also includes the steps of delivering the catheter/device assembly to an at least partially obstructed in vivo lumen location; inflating the catheter to expand the implantable medical device from the initial diameter to a second diameter; and withdrawing the catheter. A method according to the present invention may also include the step of evaluating the in vivo location of the implantable device by exposure to x-ray such that the expandable first tubular structure and the expandable second tubular structure are visible.